Multi-antenna techniques are currently being applied to several wireless systems to increase system reliability and/or system throughput. Those skilled in the art will appreciate that the highest performance gains from multi-antenna processing are obtained when multiple antennas are deployed at both ends of the wireless communication link. In best case scenarios, i.e., when channel conditions are separable between transmit and receive antennas and high signal-to-noise ratios are observed at the mobile terminal end of the communications link, two or more data streams can be transmitted simultaneously, at the same frequency, separated only in the spatial dimension. In less favorable scenarios (such as inseparable spatial channels or lower signal-to-noise ratios at the mobile terminals, etc), multi-antenna techniques can still be used to increase the link reliability via so-called spatial diversity and beamforming methods. In general, these systems with multiple antennas at both sides are referred to as Multiple-Input Multiple-Output (MIMO) systems.
The 3rd-Generation Partnership Project (3GPP) is currently developing specifications for a so-called Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) as part of their Long-Term Evolution (LTE) initiative to improve radio access technology. The air interface described by these specifications, commonly referred to simply as LTE or E-UTRA (Evolved UMTS Terrestrial Radio Access), is intended to assure competitiveness of 3GPP-based access technology. Multi-antenna techniques are central to the 3GPP LTE standards; LTE supports several different multi-antenna techniques in order to enable high spectral efficiencies in a wide range of scenarios. In particular, a number of precoding formats are specified in the 3GPP Release 8 specifications.
Precoding is a technique for mapping modulated symbols onto multiple antennas for transmission either for spatial multiplexing or diversity or beamforming purposes. Precoding is used in multi-antenna systems to adapt the transmission to the short-term and/or long term properties of the channel. (See, for example, 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation,” v8.4.0, available at http://www.3gpp.org/ftp/specs/html-info/36211.htm.) The basic idea is to adjust the phases and/or the amplitudes of the information carrying signals transmitted from the multiple antennas so that the transmitted signals better suit the channel conditions between the multiple transmitter antennas and multiple receiver antennas. Classical beamforming is a special case of precoding in which the phase of a single information-carrying signal is adjusted on each transmit antenna so that all the transmitted signals add constructively at the receiver. However, precoding for MIMO systems can more generally be described as multiplying a vector-valued information-carrying signal with a precoder matrix.
The precoder matrix is chosen based on information about the channel properties. These channel properties, in turn, are measured by observing received reference signals, and comparing the received reference signals with known or expected values for these references. Of course, these measurements reflect channel characteristics corresponding to the particular MIMO channel over which the reference signal was sent. Thus, association of reference signals together with multi-antenna layers is very important. Based on this association, reference signals are used for measurement of various channel-related parameters; these parameters are crucial for selection of the best precoding matrix.
In LTE systems, cell-specific reference signals (also referred to as common reference signals), are transmitted during the first and fifth OFDM symbols of each slot when normal cyclic prefix and two antenna ports are configured. The cell-specific reference signals are transmitted during the first and fourth OFDM symbols when extended cyclic prefix is used. In LTE Release 8, at most four cell-specific reference signals are supported. Essentially, one, two or four common reference signals may be transmitted in a cell. Terminals use these reference signals to perform measurements for mobility as well as for channel estimation, so that the transmitted data and control signals can be demodulated and decoded. The common reference signals are also used by each terminal in the cell to determine the number of supportable downlink signals or streams that best suit the current channel conditions, and may be used as well as to determine recommended precoding weights for the base station to use for downlink transmission. The terminals also measure and feedback channel quality indicators to the base station; these channel quality indicators may be used for scheduling and link adaptation.